What is the meaning of the phrase ''might is right''?

The phrase "might is right" refers to the idea that people who have power can do whatever they want.  This idea is often phrased a little differently as "might makes right."  In this case, the word "might" is being used in the sense of power and the word "right" can mean either what is morally right or what one has the right to do.

In human terms, when the people who have the might do something, it can automatically come to be seen as morally right even if it really isn't.  This is true partly because no one can stop someone else who has a lot of power.  And it is partly true because "the winners write the history."

In the context of this story, the phrase refers to how life plays out among animals.  Among animals, whoever is strong enough to kill its food or its rivals will survive.  In such a case, there is no question about what is good or bad -- just who has more power.